When firing the Crystal Bow, no mana is used from mana storing items, you only need enough mana to pass the checks - more than 200.
How to reproduce:

• Remove all mana storing items.
• Get a Mana Tablet with /give @p botania:manaTablet 1 0 {mana:201}.
• Get a Crystal Bow. Make sure you're not in creative.
• Try to fire.

Using Forge 12.18.3.2239, Botania r1.9-340, Baubles 1.10.2-1.3.9, JEI 1.10.2-3.14.2.401.